I'm going to post the spec charts on some brands.I also have the RS3000, and the 3 year warranty tops most others.Nick-

RS3000 Sine Wave Inverter/ChargerDesigned for RVs to power advanced onboard electronics. The RS3000 features 3000 watts of sine wave output and an advanced three-stage battery charger together with industry-standard networking capability.

SW2512MC & SW4024MC2Widely used throughout the world as a primary source of AC electricity, the SW offers sine wave, utility grade output power, high capacity battery charger, high surge current ability (inrush current), and easy installation.

In dealing with alot of refrigeration compressors, ... when voltage drops, why does the amp draw increase. ...How do theese motors know to try and keep a certain rpm?

Nick,

There are two basic types of AC motors (OK, I know electrical-engineer types can dispute this, as there are many variations on these today).

The first type, which is common in, for example, electric drills, is called a universal motor and is wound just like a DC motor, with slip-rings and commutators. These motors behave more or less like a resistive load -- if you reduce the voltage, you will reduce the power and speed of the motor. The advantage is that it is easy to control the speed, as it is a direct function of voltage, and it's not sensitive to line frequency. Also, it can be used on AC or DC.

The second major type is the synchronous motor. This motor will turn at a speed that is a direct function of the AC line frequency. Almost all three-phase motors are synchronous, and it is very easy to build a synchronous motor when you have three phases available. Basically, the motor has three (or multiples thereof) windings that are offset from each other by 120 degrees, and as each phase voltage peaks in its respective winding it will attract the corresponding pole of the rotor (overly simplistic, but that's the basic idea).

A single-phase synchronous motor presents special challenges. Something must be done to create a "phase offset" to set up a moving magnetic field for the rotor to follow in order to get the rotor moving. There are several techniques... low torque motors such as table fans use what is known as a "shaded pole" and high torque motors such as your compressors generally use a technique involving a start-up winding called "split phase." The start-up winding gets cut out of the circuit once the motor has reached synchronous speed by a centrifugal switch. Starting torque can be increased and/or starting current reduced in a split-phase motor through use of a "start capacitor".

In any case, one thing all synchronous motors have in common is that once running, the nature of electromagnetic force causes the motor to resist any force that would cause the rotor to fall out of synch with the AC current frequency. The further out of synch you try to push the rotor, the greater the motor will try to resist, and the current in the windings will increase to achieve this.

Any load on the motor is a force trying to drag the rotor out of synch, so current in the windings will increase to compensate. And, as we discussed, if the load on the motor is constant, and the AC frequency is constant, but the voltage drops, then current will also increase to compensate.

Incidentally, one thing that helps tremendously in a situation where you need to run an A/C on perhaps marginal park power, is to use a bigger cord. Every wire has a voltage drop across it that is related to length, but larger gauges have less drop. So when we are someplace where we have access only to, for example, a 15-amp circuit, but we need to run one A/C (running load of about 13 amps, once started), we forgo our 50', 10-gauge shore cord and haul out the big guns -- our 25' 6-gauge shore cord, with a dogbone on the end to connect to the 15-amp outlet. If we need more length, we add our 40' 6-gauge extension cord. Even though it's only carrying 15 amps, having the 6-gauge really cuts down on the voltage drop.

I am just moving this topic to the top so I do not lose it. I am going to have to take exception to Shawn's description of motors and I want to really think about it long and hard before I do so. LOLRichrd

Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body. But rather to skid in sideways, chocolate in one hand, a good Reisling in the other, body thoroughly used up, totally worn out and screaming: WOO HOO, what a ride

I am just moving this topic to the top so I do not lose it. I am going to have to take exception to Shawn's description of motors and I want to really think about it long and hard before I do so. LOL

See, I told you the electrical-engineer types would dispute it

Richard, I did say that I was glossing over a lot of stuff... I did not want to launch into a full-scale discussion of inductance, reluctance, magnetic flux, and the phase of the moon . Which is why I put the links in to basic motor technology sites.

But if I really got something wrong (as opposed to just really oversimplified), do let me know....

I am just moving this topic to the top so I do not lose it. I am going to have to take exception to Shawn's description of motors and I want to really think about it long and hard before I do so. LOLRichrd

I also advise to use adequate wire size but your example is a little extreme. Using the calculator on this link, shows a voltage drop of .66 volts on 50' of #10 at 13 amps. I don't think this will have significant effect on motor current.

I am just moving this topic to the top so I do not lose it. I am going to have to take exception to Shawn's description of motors and I want to really think about it long and hard before I do so. LOLRichrd

Couldn't ya look at how his name is spelled correctly while ya contemplating picking his post apart!

If you look close BK, you will note that I misspelled my name also. Since today is my 52nd wedding anniversary I will blame the errors on that. LOLRichard

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Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body. But rather to skid in sideways, chocolate in one hand, a good Reisling in the other, body thoroughly used up, totally worn out and screaming: WOO HOO, what a ride

Umm, I think the calculator on that link is for 12VDC. That's not the right formula for 120VAC. Also, while the A/C has a running load of 13A, the coach will be drawing 15A, the rated capacity of the circuit. When I run the numbers, I get a drop of 1.6%.

That is within acceptable tolerance, however, I was talking about a situation where the park voltage was marginal to begin with. So if you had only, say, 110 volts at the pedestal, now you'll have 108 (or maybe much, much less -- see below). You are right, that won't have too much effect on compressor running current (an extra .2 amp from the draw at the nominal rating of 120 volts), but it is, in my experience, a noticeable difference. And, with only ~2 amps left over to run everything in the coach, including the battery charger, that .2 amp represents 10% of the available capacity.

By contrast, 25' of 6-gauge is a drop of only 0.3%, less than a fifth the drop of my 10-gauge cord.

The numbers get much more definitive at higher currents. I can use the 10-gauge cord set on a 30-amp circuit as well, and I often do (although I also often dial the draw back down to 20 amps). At a 30-amp draw, the drop on 50' of #10 rises to 3.1%, considered just out of the acceptable range. If I have a solid 120VAC at the pole, I don't worry too much that I'll be getting only 116 in the coach. But if I have only 110 coming in, that will drop to 106.6, again a noticeable difference.

Remember, also, that voltage drop is current dependent. So when you take your (almost zero current) DVM to the pedestal and measure, say, 115VAC, you may not realize that the 30-A circuit on that pedestal runs back to the main panel on perhaps 100' or more of #10. So now my 50' cord is making a run of 150' on #10 wire, for a whopping 9.3% voltage drop (104 volts at the coach) on a 30-amp draw, or 4.7% on a 15-amp draw (109.6 volts). There is not much you can do about the gauge and length of the wire from the main to the pedestal, but you can at least minimize the additional drop in your additional cord.

Sean and all, In my experience, the majority of motors (excluding the hand tool motors) are asynchronous as opposed to synchronous.

Three phase Synchronous motors are designed to run at a specific speed (rpm) regardless of input voltage or the connected load, as long as these items are within the operating parameters of the motor.

For example, a four pole synchronous motor, connected to a 60 hertz power source will rotate at exactly 1800 rpm +-0 from no load to full load. Typically it will stay locked in up to 150% load and with input voltage varying as much as 20% or more. With a properly adjusted exciter, the power factor will be 1.0 pf. This means that every amp going into the motor is being converted to watts. If the excitation is increased the pf will go leading and the motor will began acting as a synchronous condenser and this phenomenon is utilized by many large companies to try and correct their normal lagging power factor, since the utility company penalizes companies for having a poor (lagging) power factor.

In my experience, less than 1% of all the 3 phase motors in existence are of the synchronous type. In fact they were so scarce that in the 70’s I had to develop (invent) a method of converting a synchronous alternator into a synchronous motor in order to have a supply of motors to build the power converters I was manufacturing for the main frame computer market place.

Many years ago these motors were used in large conveyor systems to maintain the synchronous speed of various production lines but I believe the advent of the easily controlled Variable Frequency Drive Systems driving asynchronous motors have replaced the synchronous motor.

The only other place that I am aware of this type motor being used is in the electric wall clock. Did anyone ever wonder why they keep such accurate time?

It is because the driving motor is a synchronous motor which is locked to the frequency of the utility power grid it is connected to. Additionally the entire power grid in the US is tied together so that phase A of the power in California is exactly in step with phase A in Maine.

Even further, the grid is so regulated that every night just before midnight the overall US grid is tweaked up or down, as necessary to make sure that the correct number of cycles have occurred within the past twenty four hours. And that boys and girls is why our wall clocks are so accurate.

The asynchronous motor is the workhorse of the industrial world and is more commonly referred to as an induction or squirrel cage motor. Its RPM is always less than the synchronous speed would be.

For example, a four pole induction motor, connected to a 60 hertz power line will operate at approximately 1790 rpm at no load and 1750 rpm at full load. The actual rpm depends on the quality of the motor and the design. Some are designed to be low slip, and some are designed to be high slip.

I could probably continue on for many pages, but have tried to limit this to the minimum that would get my opinion across. Richard

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Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body. But rather to skid in sideways, chocolate in one hand, a good Reisling in the other, body thoroughly used up, totally worn out and screaming: WOO HOO, what a ride

My brain must be addled -- of course you are right, I left out the third and most common type of AC motor, the induction motor. And, of course, the compressor motors Nick was asking about are likely induction motors. Always good to have a real heavy-duty-power guy around to set me straight.

The answer about the current is still the same -- any effort to resist the magnetic force trying to pull the rotor forward will be met with increased current in the windings.

Squirrel-cage rotors are also one of the possible mechanisms for starting synchronous motors until they reach synchronous speed.